Provided herein are compact sondes having a small diameter form factor and accurate depth measurement without sacrificing the number of water parameters capable of being measured by sensors arranged along a sensor plane.
In a small diameter submersible multiparameter sonde, space constraints and limitations can make it challenging to align all sensors to a common sensor plane. Although a pressure sensor used to measure depth is a good candidate to locate elsewhere on the sonde, challenges arise with such a configuration. For example, positioning a pressure sensor away from the common sensor plane can result in significant errors in determining the actual depth of the sensor plane as the sonde moves and orients in different directions, including non-vertical directions.
One method for determining actual depth of the sonde sensor plane is to add to the depth determined by the pressure sensor the corresponding separation distance between the sensor plane and the location of the pressure sensor. This provides an accurate depth determination, however, only for a sonde in the absolute vertical orientation. Failing to account for the deployment angle relative to vertical of the sonde can result in an inaccurate depth measurement and corresponding incorrect deployment depth of the sonde sensor plane. For example, if the sonde flips from a vertical to horizontal during use, the error in depth for such a method corresponds to the separation distance between the sensor plane and the pressure sensor. Similarly, consider a typical longitudinal separation distance between sensor plane and pressure sensor position (Ds) of 7.50 inches. If the orientation angle (θ) changes 15° (from an expected 90° to 75°), the vertical separation distance between sensor plane and pressure sensor (Dx) changes from the expected 7.50 inches to 7.24 inches, an error of 0.26 inches. In a shallow water application, for example 20 feet, that error approaches the magnitude of the error of the pressure sensor, effectively doubling the error of the system. In the case where the sonde is laying horizontal on the bottom (θ=0°), the error becomes 7.50 inches. In the extreme case where the end of the sonde is hung up on a rock and is completely upside down (θ=−90°), the error in this example can jump to 15 inches of water pressure.
Accordingly, there is a need for the ability to accurately measure depth in sondes having a pressure sensor for depth measurement that is positioned away from the sensor plane. Such a configuration provides the functional benefit of being able to maximize the number of water parameters measured at the sensor plane by avoiding space otherwise occupied by the pressure sensor, while ensuring the sonde deployment depth is accurately determined at the sensor plane.